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Vibrational Spectroscopy of SurfacesRAIRSEELSSERS

Reflection Absorption Infrared SpectroscopyElectron Energy Loss SpectroscopySurface Enhanced Raman Spectroscopy

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IR Spectroscopy: Review

transitions are excited by IR radiation (which is absorbed)-causes molecule to vibrate

vibrational frequency governed by force constant (slope of potentialenergy curve.

Normal Modes � identify particular types of vibrations-actually mixed but a reasonable approximation

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Intensities

Intensity is proportional to the square of the derivative dipole moment

requires -Einstein Coefficients of absorption and emission- Plank's radiation law-perturbation theory

for a simple harmonic oscillator selection rule is v="1

absorption occurs if there is a change in dipole moment associated with the vibration

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Instrument in simple form

Notice that the beam strikes the sample at a glancingincident angle: why?

(FT-IR)

recall CHEM 3440

we will see soon

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First Studies were not done like that (1958)Eischens and Pliskin used thin wafers of metals supported on silicaMetals were produced by soaking silica in salt of metal and then reducing inhydrogen at high temperature

*IR Cell is enlarged

CaF2

Gas in

Metal/SiO2 film on CaF2 disk

Dispersive IR Spectrometer (No FT-IR)

"X-Y recorder"

IR source

(no computers)CaF2

Quartzfurnace

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H2PtCl6 hydrogen reduction

"finely divided metal catalysts"

Pt/SiO2, PdSiO2

Spectra of Carbon Monoxide adsorption

increasingcoverage

fostered ideas ofbridging and linearlybonded CO: see next page

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Interpreted in terms of "model" cluster compounds

Octahedral complexes, e.g. Cr(CO)6, exhibits only a single νCO band but the IR spectrum of Fe2(CO)9 displays CO bands at 2082, 2019, 1829 cm-1.In cluster carbonyls, νCO is a sensitive probe for the CO coordination geometry. For bridging (µ2) ligandsνCO is usually shifted by 100-200 cm-

1 to lower wavenumbers compared to the signatures of µ1-CO. Bands for face capping (µ3) CO ligands appear at even lower energies.

18192045, 2075Rh6(CO)16

18862044, 2070, 2074Rh4(CO)12

1846, 18622060, 2084Rh2(CO)8

νCO, µ3 (cm-1)νCO, µ2 (cm-1)νCO, µ1 (cm-1)carbonyl

gas phase CO : 2143 cm-1

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Samples above are heterogeneous in the sense that many crystalplanes are exposed.

Thrust was then to study single crystal surfaces: recall LEEDand STM/AFM

This ushered in reflection methods

we now develop the associated theory

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Theory of RAIRSgiven sound basis by Greenler (J. Chem. Phys. 44 (1966) 310 and 50 (1969) 1963

perp ppar (s)

polarization iswith respect to plane of incidence

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for IR radiation polarized perp toplane of incidence (par to surface)the 180 degree phase changegives zero electric field at surface-no IR absorption can occur

i.e., in IR distance from node to antinodeis about 10,000 D: monolayer is 10 D (or less)

"If the electric field has no amplitudeat the metal surface, it can not interact withany molecules adsorbed on the metal surface. In the5:. (2000cm-l ) infrared spectral region, the distancebetween the node and adjacent anti node of the standingwave is 12500 D, which is more than 2000 times thethickness of a 5D layer of adsorbed material. It wouldappear that the surface of the metal would be an excellentplace to locate a thin layer of material which wasto be hidden from detection by the infrared spectroscopist."

Greenler said:

for p-polarized light phase change depends stronglyon incident angle.

shows how electric field vectoroscillates for incident and reflected rays

-elliptical standing wave

note this: see later

but

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copper (2100 cm-1)

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similar curves existfor other metals

optimum is grazing incidencefor one reflection

Summary

-only p-polarized light can- interact with surface dipoles-best sensitivity is achieved for- glancing incidence angles- only vibrations with a component- of a dipole moment changeperpendicular to surface

will be active

Dependence of square of the electric field amplitude E of the stationary wave on angle of incidence

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Alternative Approach "Surface Selection Rule"

vibrations parallel to surface involve no dipole moment change-dipoles cancel out.

7-15important for EELS�see later